Locally dimmed display

ABSTRACT

A locally dimmed display has a spatial light modulator illuminated by a light source. The spatial light modulator is illuminated with a low resolution version of a desired image. The illumination may comprise a series of lighting elements that vary smoothly from one element to another at the spatial light modulator.

The claimed invention was made as a result of activities undertakenwithin the scope of a joint research agreement as defined under 35U.S.C. 103(c) between the National Sciences and Engineering ResearchCouncil of Canada, Lorne Whitehead of the University of BritishColumbia, Wolfgang Stuerzlinger and Hugh Wilson of York University, andAvi Chaudhuri of McGill University.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/097,823 filed 29 Apr. 2011, which is a continuation of U.S. patentapplication Ser. No. 12/855,918 filed on 13 Aug. 2010, which is acontinuation of U.S. patent application Ser. No. 12/410,125 filed on 24Mar. 2009, which is a continuation of U.S. patent application Ser. No.11/831,749 filed on 31 Jul. 2007, which is a continuation of U.S. patentapplication Ser. No. 11/702,839 filed on 5 Feb. 2007, which is acontinuation of U.S. patent application Ser. No. 11/351,962 filed on 10Feb. 2006, which is a continuation of U.S. patent application Ser. No.11/112,428 filed on 22 Apr. 2005, which is a divisional of U.S. patentapplication Ser. No. 10/469,473 (accorded the filing date of 27 Aug.2003), which is the U.S. National Stage of International Application No.PCT/CA2002/000255 filed on 27 Feb. 2002 and entitled HIGH DYNAMIC RANGEDISPLAY DEVICES, which claims the benefit of the filing date of U.S.provisional patent application No. 60/271,563 filed on 27 Feb. 2001 andentitled HIGH DYNAMIC RANGE COLOUR DISPLAY AND PROJECTION TECHNOLOGY.

TECHNICAL FIELD

The invention relates to displays for displaying digital images.

BACKGROUND

Dynamic range is the ratio of intensity of the highest luminance partsof a scene and the lowest luminance parts of a scene. For example, theimage projected by a video projection system may have a maximum dynamicrange of 300:1.

The human visual system is capable of recognizing features in sceneswhich have very high dynamic ranges. For example, a person can look intothe shadows of an unlit garage on a brightly sunlit day and see detailsof objects in the shadows even though the luminance in adjacent sunlitareas may be thousands of times greater than the luminance in the shadowparts of the scene. To create a realistic rendering of such a scene canrequire a display having a dynamic range in excess of 1000:1. The term“high dynamic range” means dynamic ranges of 800:1 or more.

Modern digital imaging systems are capable of capturing and recordingdigital representations of scenes in which the dynamic range of thescene is preserved.

Computer imaging systems are capable of synthesizing images having highdynamic ranges. However, current display technology is not capable ofrendering images in a manner which faithfully reproduces high dynamicranges.

Blackham et al., U.S. Pat. No. 5,978,142 discloses a system forprojecting an image onto a screen. The system has first and second lightmodulators which both modulate light from a light source. Each of thelight modulators modulates light from the source at the pixel level.Light modulated by both of the light modulators is projected onto thescreen.

Gibbon et al., PCT application No. PCT/US01/21367 discloses a projectionsystem which includes a pre modulator. The pre modulator controls theamount of light incident on a deformable mirror display device. Aseparate pre-modulator may be used to darken a selected area (e.g. aquadrant).

There exists a need for cost effective displays capable of reproducing awide range of light intensities in displayed images.

SUMMARY OF THE INVENTION

This invention provides displays for displaying images and methods fordisplaying images. One aspect of the invention provides a displaycomprising: a light source; a first spatial light modulator located tomodulate light from the light source; a display screen comprising asecond spatial light modulator; and, an optical system configured toimage light modulated by the first spatial light modulator onto a firstface of the display screen.

Another aspect of the invention provides a display comprising: a lightsource; a first spatial light modulator located to modulate light fromthe light source, the first spatial light modulator comprising an arrayof controllable pixels; and, a second spatial light modulator located tomodulate light modulated by the first spatial light modulator the secondspatial light modulator comprising an array of controllable pixels;wherein each pixel of one of the first and second spatial lightmodulators corresponds to a plurality of pixels of the other one of thefirst and second light modulators.

Another aspect of the invention provides a display device comprising:first spatial modulation means for providing light spatially modulatedat a first spatial resolution; second spatial modulation means forfurther spatially modulating the light at a second resolution differentfrom the first resolution; and, means for controlling the first andsecond spatial modulation means to display an image defined by imagedata.

A still further aspect of the invention provides a method for displayingan image having a high dynamic range. The method comprises: generatinglight, spatially modulating the light according to image data in a firstlight modulating step; and, imaging the spatially modulated light onto ascreen comprising a light modulator.

Further aspects of the invention and features of specific embodiments ofthe invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate non-limiting embodiments of the invention,

FIG. 1 is a schematic illustration of a display according to oneembodiment of the invention;

FIG. 1A is a schematic illustration of a specific implementation of thedisplay of FIG. 1;

FIG. 2 is a schematic illustration of a display according to analternative embodiment of the invention comprising four spatial lightmodulators;

FIG. 3 is a schematic illustration of a rear-projection-type displayaccording to a further embodiment of the invention;

FIG. 4 is a schematic illustration of a front-projection-type displayaccording to a still further embodiment of the invention;

FIG. 5 is a drawing illustrating a possible relationship between pixelsin a higher-resolution spatial light modulator and pixels in alower-resolution spatial light modulator in a display according to theinvention;

FIG. 5A illustrates an effect of providing one light modulator which haslower resolution than another light modulator;

FIG. 6 is a schematic illustration of a front-projection-type colordisplay having an alternative projector construction;

FIGS. 6A and 6B are expanded cross-sectional views of portions of thefront-projection screen of the color display of FIG. 6;

FIG. 7 is a graph illustrating how light imaged onto a higher-resolutionlight modulator from pixels of a lower-resolution light modulator canoverlap to yield a smooth variation in light intensity with position;and,

FIG. 7A is a graph illustrating how the variation in light intensitywith position for the image of a pixel of a light modulator can berepresented as the convolution of a square profile and a spreadfunction.

DESCRIPTION

Throughout the following description, specific details are set forth inorder to provide a more thorough understanding of the invention.However, the invention may be practiced without these particulars. Inother instances, well known elements have not been shown or described indetail to avoid unnecessarily obscuring the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative, ratherthan a restrictive, sense.

This invention provides displays capable of rendering images with highdynamic ranges. Displays according to the invention comprise two lightmodulating stages. Light passes through the stages in series to providean image which has an increased dynamic range.

FIG. 1 illustrates schematically a display 10 according to a simpleembodiment of the invention. The sizes of elements and distances betweenthem in FIG. 1 are not to scale. Display 10 comprises a light source 12.Light source 12 may, for example, comprise a projection lamp such as anincandescent lamp or an arc lamp, a laser, or another suitable source oflight. Light source 12 may comprise an optical system comprising one ormore mirrors, lenses or other optical elements which cooperate todeliver light to the rest of display 10.

In the illustrated embodiment, light from light source 12 is directedtoward a first light modulator 16. Light source 12 preferably providessubstantially uniform illumination of first light modulator 16. Lightmodulator 16 comprises an array of individually addressable elements.Light modulator 16 may comprise, for example, a LCD (liquid crystaldisplay), which is an example of a transmission-type light modulator ora DMD (deformable mirror device), which is an example of areflection-type light modulator. Display driver circuitry (not shown inFIG. 1) controls the elements of light modulator 16 according to datawhich defines an image being displayed.

Light which has been modulated by first light modulator 16 is imagedonto a rear-projection screen 23 by a suitable optical system 17. Lightfrom a small area of first light modulator 16 is directed by opticalsystem 17 to a corresponding area on rear-projection screen 23. In theillustrated embodiment, optical system 17 comprises a lens having afocal length f. In general, the optical system 17 which images lightmodulated by first light modulator 16 onto rear-projection screen 23 maycomprise one or more mirrors, lenses or other optical elements. Such anoptical system has the function of imaging light modulated by the firstlight modulator onto a second light modulator.

In the illustrated embodiment, rear-projection screen 23 comprises asecond light modulator 20 and a collimator 18. A main function ofcollimator 18 is to cause light which passes through rear-projectionscreen 23 to be directed preferentially toward a viewing area.Collimator 18 may comprise a Fresnel lens, a holographic lens, or, inthe alternative, another arrangement of one or more lenses and/or otheroptical elements which will guide light in the direction of a viewingarea.

In the illustrated embodiment, collimator 18 causes light to travelthrough the elements of second light modulator 20 in a direction whichis generally normal to screen 23. As light incident from collimator 18travels through second light modulator 20 it is further modulated. Thelight then passes to a diffuser 22 which scatters the outgoing lightthrough a range of directions so that a viewer located on an oppositeside of diffuser 22 from first light modulator 16 can see lightoriginating from the whole area of screen 23. In general, diffuser 22may scatter light to a different angular extent in the horizontal andvertical planes. Diffuser 22 should be selected so that light modulatedby second light modulator 20 is scattered through a range of angles suchthat the maximum scatter angle is at least equal to the angle subtendedby screen 23 when viewed from a desired viewing location.

Rear-projection screen 23 may differ in area from first light modulator16. For example, rear-projection screen 23 may be larger in area thanfirst light modulator 16. Where this is the case, optical system 17expands the beam of light modulated by first light modulator 16 toilluminate a larger corresponding area on rear-projection screen 23.

Second light modulator 20 may be of the same type as first lightmodulator 16 or a different type. Where first and second lightmodulators 16 and 20 are both of types that polarize light, second lightmodulator 20 should, as much as is practical, be oriented so that itsplane of polarization matches that of the light incident on it fromfirst light modulator 16.

Display 10 may be a color display. This may be achieved in various waysincluding:

-   -   making one of first light modulator 16 and second light        modulator 20 a color light modulator;    -   providing a plurality of different first light modulators 16        operating in parallel on different colors; and,    -   providing a mechanism for rapidly introducing different color        filters into the light path ahead of second light modulator 20.

As an example of the first approach above, second light modulator 20 maycomprise an LCD panel having a plurality of pixels each comprising anumber of colored sub-pixels. For example, each pixel may comprise threesub-pixels, one associated with a red filter, one associated with agreen filter and one associated with a blue filter. The filters may beintegral with the LCD panel.

As shown in FIG. 1A, Light source 12, first light modulator 16 andoptical system 17 may all be parts of a digital video projector 37located to project an image defined by a signal 38A from a controller 39onto the back side of rear-projection screen 23. The elements of secondlight modulator 20 are controlled by a signal 38B from controller 39 toprovide an image to a viewer which has a high dynamic range.

As shown in FIG. 2, a display 10A according to the invention maycomprise one or more additional light modulation stages 24. Eachadditional light modulation stage 24 comprises a collimator 25, a lightmodulator 26 and an optical system 27 which focuses light from lightmodulator 26 onto either the next additional light modulation stage 24or on collimator 18. In device 10A of FIG. 2 there are two additionallight modulation stages 24. Devices according to this embodiment of theinvention may have one or more additional light modulation stages 24.

The luminance of any point on output diffuser 22 can be adjusted bycontrolling the amount of light passed on by corresponding elements oflight modulators 16, 20 and 26. This control may be provided by asuitable control system (not shown in FIG. 2) connected to drive each oflight modulators 16, 20 and 26.

As noted above, light modulators 16, 20 and 26 may all be of the sametype or may be of two or more different types. FIG. 3 illustrates adisplay 10B according to an alternative embodiment of the inventionwhich includes a first light modulator 16A which comprises a deformablemirror device. A deformable mirror device is a “binary” device in thesense that each pixel may be either “on” or “off”. Different apparentbrightness levels may be produced by turning a pixel on and off rapidly.Such devices are described, for example, in U.S. Pat. Nos. 4,441,791and, 4,954,789 and are commonly used in digital video projectors. Lightsource 12 and first light modulator 16 (or 16A) may be the light sourceand modulator from a commercial digital video projector, for example.

FIG. 4 illustrates a front-projection-type display 10C according to theinvention. Display 10C comprises a screen 34. A projector 37 projects animage 38 onto screen 34. Projector 37 comprises a suitable light source12, a first light modulator 16 and an optical system 17 suitable forprojecting an image defined by first light modulator 16 onto screen 34.Projector 37 may comprise a commercially available display projector.Screen 34 incorporates a second light modulator 36. Second lightmodulator 36 comprises a number of addressable elements which can beindividually controlled to affect the luminance of a corresponding areaof screen 34.

Light modulator 36 may have any of various constructions. For example,light modulator 36 may comprise an array of LCD elements each having acontrollable transmissivity located in front of a reflective backing.Light projected by projector 37 passes through each LCD element and isreflected back through the LCD element by the reflective backing. Theluminance at any point on screen 34 is determined by the intensity oflight received at that point by projector 37 and the degree to whichlight modulator 36 (e.g. the LCD element at that point) absorbs lightbeing transmitted through it.

Light modulator 36 could also comprise an array of elements havingvariable retro-reflection properties. The elements may be prismatic.Such elements are described, for example, in Whitehead, U.S. Pat. No.5,959,777 entitled Passive High Efficiency Variable Reflectivity ImageDisplay Device and, Whitehead et al., U.S. Pat. No. 6,215,920 entitledElectrophoretic, High Index and Phase Transition Control of TotalInternal Reflection in High Efficiency Variable Reflectivity ImageDisplays.

Light modulator 36 could also comprise an array of electrophoreticdisplay elements as described, for example, in Albert et al., U.S. Pat.No. 6,172,798 entitled Shutter Mode Microencapsulated ElectrophoreticDisplay; Comiskey et al., U.S. Pat. No. 6,120,839 entitledElectro-osmotic Displays and Materials for Making the Same; Jacobson,U.S. Pat. No. 6,120,588 entitled: Electronically AddressableMicroencapsulated Ink and Display; Jacobson et al., U.S. Pat. No.6,323,989 entitled Electrophoretic Displays Using Nanoparticles; Albert,U.S. Pat. No. 6,300,932 entitled Electrophoretic Displays withLuminescent Particles and Materials for Making the Same or, Comiskey etal., U.S. Pat. No. 6,327,072 entitled Microcell ElectrophoreticDisplays.

As shown in FIGS. 6A and 6B, screen 34 preferably comprises a lenselement 40 which functions to direct light preferentially toward theeyes of viewers. In the illustrated embodiment, lens element 40comprises a Fresnel lens having a focal point substantially coincidentwith the apex of the cone of light originating from projector 37. Lenselement 40 could comprise another kind of lens such as a holographiclens. Lens element 40 incorporates scattering centers 45 which provide adesired degree of diffusion in the light reflected from screen 34. Inthe illustrated embodiment, second light modulator 36 comprises areflective LCD panel having a large number of pixels 42 backed by areflective layer 43 and mounted on a backing 47.

Where light modulator 36 comprises an array of elements having variableretro-reflection properties, the elements themselves could be designedto direct retro-reflected light preferentially in a direction of aviewing area in front of screen 34. Reflective layer 43 may be patternedto scatter light to either augment the effect of scattering centers 45or replace scattering centers 45.

As shown in FIG. 4, a controller 39 provides data defining image 38 toeach of first light modulator 16 and second light modulator 36.Controller 39 could comprise, for example, a computer equipped with asuitable display adapter. Controller 39 may comprise image processinghardware to accelerate image processing steps. The luminance of anypoint on screen 34 is determined by the combined effect of the pixels infirst light modulator 16 and second light modulator 36 which correspondto that point. There is minimum luminance at points for whichcorresponding pixels of the first and second light modulators are set totheir “darkest” states. There is maximum luminance at points for whichcorresponding pixels of the first and second light modulators are set totheir “brightest” states. Other points have intermediate luminancevalues. The maximum luminance value might be, for example, on the orderof 10⁵ cd/m². The minimum luminance value might be, for example on theorder of 10⁻² cd/m².

The cost of a light modulator and its associated control circuitry tendsto increase with the number of addressable elements in the lightmodulator. In some embodiments of the invention one of the lightmodulators has a spatial resolution significantly higher than that ofone or more other ones of the light modulators. When one or more of thelight modulators are lower-resolution devices the cost of a displayaccording to such embodiments of the invention may be reduced. In colordisplays comprising two or more light modulators, one of which is acolor light modulator (a combination of a plurality of monochrome lightmodulators may constitute a color light modulator as shown, for example,in FIG. 6) and one of which is a higher-resolution light modulator, thehigher-resolution light modulator should also be the color lightmodulator. In some embodiments the higher-resolution light modulator isimaged onto the lower-resolution light modulator. In other embodimentsthe lower-resolution light modulator is imaged onto thehigher-resolution light modulator.

FIG. 5 illustrates one possible configuration of pixels in a display 10as shown in FIG. 1. Nine pixels 42 of a second light modulator 20correspond to each pixel 44 of a first light modulator 16. The number ofpixels 42 of second light modulator 20 which correspond to each pixel 44of first light modulator 16 may be varied as a matter of design choice.Pixels 44 of the higher-resolution one of first and second lightmodulators 16 and 20 (or 36) should be small enough to provide a desiredoverall resolution. In general there is a trade off between increasingresolution and increasing cost. In a typical display thehigher-resolution light modulator will provide an array of pixels havingat least a few hundred pixels in each direction and more typically over1000 pixels in each direction.

The size of pixels 42 of the lower-resolution one of the first andsecond light modulators determines the scale over which one can reliablygo from maximum intensity to minimum intensity. Consider, for example,FIG. 5A which depicts a situation where one wishes to display an imageof a small maximum-luminance spot on a large minimum-luminancebackground. To obtain maximum luminance in a spot 47, those pixels ofeach of the first and second light modulators which correspond to spot47 should be set to their maximum-luminance values. Where the pixels ofone light modulator are lower in resolution than pixels of the otherlight modulator then some pixels of the lower-resolution light modulatorwill straddle the boundary of spot 47. This is the case, for example, inFIG. 5A.

Outside of spot 47 there are two regions. In region 48 it is notpossible to set the luminance to its minimum value because in thatregion the lower-resolution light modulator is set to its highestluminance value. In region 49 both of the light modulators can be set totheir lowest-luminance values. If, for example, each of the first andsecond light modulators has a luminance range of 1 to 100 units, thenregion 47 might have a luminance of 100×100=10,000 units, region 48would have a luminance of 100×1=100 units and region 49 would have aluminance of 1×1=1 units.

As a result of having one of the light modulators lower in resolutionthan the other, each pixel of the lower-resolution light modulatorcorresponds to more than one pixel in the higher-resolution lightmodulator. It is not possible for points corresponding to any one pixelof the lower-resolution light modulator and different pixels of thehigher-resolution light modulator to have luminance values at extremesof the device's dynamic range. The maximum difference in luminancebetween such points is determined by the dynamic range provided by thehigher-resolution light modulator.

It is generally not a problem that a display is not capable of causingclosely-spaced points to differ in luminance from one another by thefull dynamic range of the display. The human eye has enough intrinsicscatter that it is incapable of appreciating large changes in luminancewhich occur over very short distances in any event.

In a display according to the invention which includes both alower-resolution spatial light modulator and a higher-resolution spatiallight modulator, controller 39 may determine a value for each pixel ofthe lower-resolution spatial light modulator and adjust the signalswhich control the higher-resolution spatial light modulator to reduceartifacts which result from the fact that each pixel of thelower-resolution spatial light modulator is common to a plurality ofpixels of the higher-resolution spatial light modulator. This may bedone in any of a wide number of ways.

For example, consider the case where each pixel of the lower-resolutionspatial light modulator corresponds to a plurality of pixels of thehigher-resolution spatial light modulator. Image data specifying adesired image is supplied to the controller. The image data indicates adesired luminance for an image area corresponding to each of the pixelsof the higher-resolution spatial light modulator. The controller may setthe pixels of the lower-resolution light modulator to provide anapproximation of the desired image. This could be accomplished, forexample, by determining an average or weighted average of the desiredluminance values for the image areas corresponding to each pixel of thelower-resolution light modulator.

The controller may then set the pixels of the higher-resolution lightmodulator to cause the resulting image to approach the desired image.This could be done, for example, by dividing the desired luminancevalues by the known intensity of light incident from thelower-resolution light modulator on the corresponding pixels of thehigher-resolution light modulator. Processing to generate the signalsfor driving the light modulators may be performed on the fly bycontroller 39, may be performed earlier by controller 39 or some otherdevice and integrated into the image data or some processing may beperformed earlier and controller 39 may perform final processing togenerate the control signals.

If the low-resolution pixels are too large then a viewer may be able todiscern a halo around bright elements in an image. The low resolutionpixels are preferably small enough that the appearance of bright patcheson dark backgrounds or of dark spots on bright backgrounds is notunacceptably degraded. It is currently considered practical to providein the range of about 8 to about 144, more preferably about 9 to 36,pixels on the higher-resolution light modulator for each pixel of thelower-resolution light modulator.

The sizes of steps in which each of pixels 42 and 44 can adjust theluminance of point(s) on the image are not necessarily equal. The pixelsof the lower-resolution light modulator may adjust light intensity incoarser steps than the pixels of the higher-resolution light modulator.For example, the lower-resolution light modulator may permit adjustmentof light intensity for each pixel over an intensity range of 1 to 512units in 8 steps while the higher-resolution light modulator may permitadjustment of the light intensity for each pixel over a similar range in512 steps. While pixels 42 and 44 are both illustrated as being squarein FIG. 5, this is not necessary. Pixels 42 and/or 44 could be othershapes, such as rectangular, triangular, hexagonal, round, or oval.

The pixels of the lower-resolution light modulator preferably emit lightwhich is somewhat diffuse so that the light intensity varies reasonablysmoothly as one traverses pixels of the lower-resolution lightmodulator. This is the case where the light from each of the pixels ofthe lower-resolution light modulator spreads into adjacent pixels, asshown in FIG. 7. As shown in FIG. 7A, the intensity profile of a pixelin the lower-resolution light modulator can often be approximated bygaussian spread function convolved with a rectangular profile having awidth d₁ equal to the active width of the pixel. The spread functionpreferably has a full width at half maximum in the range of 0.3×d₂ to3×d₂, where d₂ is the center-to-center inter-pixel spacing, to yield thedesired smoothly varying light intensity. Typically d₁ is nearly equalto d₂.

In the embodiment of FIG. 5, each pixel 42 comprises three sub pixels43R, 43G and 43B (for clarity FIG. 5 omits sub pixels for some pixels42). Sub-pixels 43R, 43G and 43B are independently addressable. They arerespectively associated with red, green and blue color filters which areintegrated into second light modulator 20. Various constructions of LCDpanels which include a number of colored sub-pixels and are suitable foruse in this invention are known in the art.

For front projection-type displays (for example the display 10C of FIG.4), it is typically most practical for first light modulator 16 tocomprise a high-resolution light modulator which provides colorinformation and for light modulator 36 to comprise a monochrome lightmodulator. Light modulator 36 preferably has reasonably smalladdressable elements so that the boundaries of its elements do not forma visually distracting pattern. For example, light modulator 36 may havethe same number of addressable elements as projector 37 (although eachsuch element will typically have significantly larger dimensions thanthe corresponding element in light modulator 16 of projector 37).

Projector 37 may have any suitable construction. All that is required isthat projector 37 be able to project light which has been spatiallymodulated to provide an image onto screen 34. FIG. 6 illustrates adisplay system 10D according to a further alternative embodiment of theinvention. System 10D comprises a screen 34 which has an integratedlight modulator 36 as described above with reference to FIG. 4. System10D comprises a projector 37A which has separate light modulators 16R,16G and 16R for each of three colors. Light modulated by each of lightmodulators 16R, 16G and 16R is filtered by a corresponding one of threecolored filters 47R, 47G and 47B. The modulated light is projected ontoscreen 34 by optical systems 17. A single light source 12 may supplylight to all three light modulators 16R, 16G, and 16B, or separate lightsources (not shown) may be provided.

As will be apparent to those skilled in the art in the light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof. For example:

-   -   diffuser 22 and collimator 18 could be combined with one        another;    -   diffuser 22 and collimator 18 could be reversed in order;    -   multiple cooperating elements could be provided to perform light        diffusion and/or collimation;    -   the order in screen 23 of second light modulator 20 collimator        18 and diffuser 22 could be varied;    -   the signal 38A driving first light modulator 16 may comprise the        same data driving second light modulator 20 or may comprise        different data.        Accordingly, the scope of the invention is to be construed in        accordance with the substance defined by the following claims.

1. A display comprising: a controller configured to receive image dataand to control elements of a low-resolution light source and elements ofa light-modulating panel having a spatial resolution higher than that ofthe light source, the image data specifying a desired image, the desiredimage having a desired luminance corresponding to each of the pixels ofthe light-modulating panel; the light source connected to receive fromthe controller first signals for separately controlling light output ofa plurality of separately-controllable elements of the light source andconfigured such that light from the elements of the light sourceilluminates a face of the light modulating panel, wherein light from theelements of the light source spreads and mixes with light from adjacentones of the elements of the light source such that output light variessmoothly; the light-modulating panel connected to receive from thecontroller second signals for controlling light transmissivity of pixelsof the light-modulating panel; the controller configured to determinefrom the image data driving values for the separately-controllableelements of the light source and to provide the driving values in thefirst signals such that application of the first signals to control theseparately-controllable elements of the light source results inillumination of the light modulating panel with light that provides anapproximation of the desired image having a variation in light intensitywith position on the light-modulating panel according to the image data,the controller further configured to set the second signals forcontrolling light transmissivity of the pixels of the light-modulatingpanel such that the resulting displayed image approaches the desiredimage specified by the image data.
 2. A display according to claim 1wherein the controller is configured to determine the light intensitiesof the approximation at the pixels of the light-modulating panel byestimating intensities of light emitted from the light source layer atthe pixels of the light-modulating panel.
 3. A display according toclaim 2 wherein the controller is configured to determine the drivingvalues for the separately-controllable elements of the light sourcelayer by determining averages or weighted averages of the desiredluminance values for image areas corresponding to each of the elementsof the light source layer.
 4. A display according to claim 1 wherein thecontroller is configured to set the pixels of the light-modulating panelbased on elements of the light source having a spread function with afull width at half-maximum in the range of 0.3×d2 to 3×d2, where d2 is acenter-to-center spacing of the separately-controllable elements of thelight source.
 5. A display according to claim 4 wherein the spreadfunction has a full width at half-maximum approximately equal to d2. 6.A display according to claim 1 wherein the controller is configured toestimate an approximate intensity of light from the elements of thelight source and to use the estimate to set the pixels of thelight-modulating panel.
 7. A display according to claim 1 wherein thelight from the elements of the light source spreads substantially to thecenter of the adjacent pixels.
 8. A display according to claim 1 whereinthe display has a dynamic range of 800:1 or greater.
 9. A displayaccording to claim 1 wherein all elements of the light source have avariable light intensity controlled by the controller.
 10. A displayaccording to claim 1 wherein the light source is configured to emitwhite light.
 11. A display according to claim 1 wherein thelight-modulating panel comprises a color LCD panel wherein the pixels ofthe light-modulating panel comprise colur sub-pixels.
 12. A displayaccording to claim 1 wherein a maximum luminance of the display is atleast 10⁵ cd/m².
 13. A display according to claim 1 wherein the elementsor the light source are rectangular.
 14. A display according to claim 1wherein the elements of the light source are configured to emit lightwhich is somewhat diffuse.
 15. A display according to claim 1 wherein anintensity profile of light from an element of the light source has apeak and falls off smoothly from the peak.
 16. A display according toclaim 15 wherein the intensity profile comprises a Gaussian spreadfunction.
 17. A display controller for controlling a display having acontrollable light source and a light-modulating panel illuminated bythe light source the light-modulating panel having a spatial resolutionhigher than that of the light source, the controller configured toreceive image data specifying a desired image and to generate firstsignals for separately controlling light output of a plurality ofseparately-controllable elements of the light source and second signalsfor controlling light transmissivity of the pixels of thelight-modulating panel, the desired image having a desired luminancecorresponding to each of the pixels of the light-modulating panel; thecontroller configured to determine from the image data driving valuesfor the separately-controllable elements of the light source such thatapplication of the driving values to drive the separately-controllableelements of the light source results in light from the elements of thelight source illuminating a face of the light modulating panel, whereinthe light from the elements of the light source spreads and mixes withlight from adjacent ones of the elements of the light source such thatemitted light varies smoothly to provide an approximation of the desiredimage having a variation in light intensity with position on thelight-modulating panel according to the image data; the controllerfurther configured to set the signals for controlling lighttransmissivity of the pixels of the light-modulating panel based on aknown intensity of light from the light source on the pixels such thatthe resulting displayed image approaches the desired image specified bythe image data.
 18. A controller according to claim 17 wherein thecontroller is configured to determine the light intensities of theapproximation at the pixels of the light-modulating panel by estimatingintensities of light emitted from the light source layer at the pixelsof the light-modulating panel.
 19. A controller according to claim 18wherein the controller is configured to determine the driving values forthe separately-controllable elements of the light source layer bydetermining averages or weighted averages of the desired luminancevalues for image areas corresponding to each of the elements of thelight source layer.
 20. A controller according to claim 17 wherein thecontroller is configured to set the pixels of the light-modulating panelbased on elements of the light source having a spread function with afull width at half-maximum in the range of 0.3×d2 to 3×d2, where d2 is acenter-to-center spacing of the separately-controllable elements of thelight source.
 21. A controller according to claim 20 wherein the spreadfunction has a full width at half-maximum approximately equal to d2. 22.A controller according to claim 17 wherein the controller is configuredto estimate an approximate intensity of light from the elements of thelight source and to use the estimate to set the pixels of thelight-modulating panel.